There are a number of neurological events and conditions which are characterized by abnormal neural-electric activity in the brain including epilepsy, migraine headaches and even some forms of depression. Epilepsy is a disease characterized by recurrent unprovoked seizures which result in episodic impairment or loss of consciousness, abnormal motor phenomena, psychic or sensory disturbances, or the perturbation of the autonomic nervous system. It is caused by abnormal firing of neurons in the brain, a condition known as epileptogenesis. These abnormal firings or electrical discharges may start in small neuronal populations (these are known as epileptogenic foci, the condition defined as focal epilepsy) or much larger areas of the brain (this condition is defined as generalized epilepsy). Often there can be a period of abnormal firing of neurons which precedes the full blown seizure. This period is known as a pre-seizure state and it can include one or more events of abnormal firing, known as pre-seizure events.
Whatever the cause, the human and financial impact of the disease is significant. The prevalence of epilepsy in the US is currently about three million worldwide; about fifty million with 200,000 new cases are diagnosed each year in the US alone. Ten percent of the American population will experience a seizure in their lifetimes. Due to the impairing nature of epileptic seizures, the disease can prevent patients from performing a number of routine activities including driving a car or operating machinery. Many states put driving restrictions on those diagnosed with epilepsy. In a sub-population of patients, the severity of the disease is so extreme that they are essentially incapacitated. The economic cost of the disease is estimated to be $12.5 billion per year in direct and indirect costs.
While there are a number of available drug therapies, these therapies have a number of side effects including hyperplasia, slurred speech and memory loss. They also require precise control of the therapeutic dosage to avoid occurrence of seizures for too low a dose or side effects for too high a dose. Also estimates are that at least 20-30 percent of epilepsy patients cannot be effectively treated with currently available drug therapies. Many persons having medically refractory epilepsy with partial-onset seizures are known not to respond well to anti seizure medication. The only option for these and other patients is radical brain surgery which presents significant mortality issues. While there have been various attempts at using electrical stimulation of the brain, particularly deep brain stimulation as a means of treating the disease, these approaches are limited to the use of continuous stimulation and do not employ detection means so as to modulate or otherwise modify the stimulation causing a change in the patient's brain activity. Also continuous deep brain stimulation has several drawbacks. To be effective, the treatment may require stimulation of the neocortex, which is often the origin or focus of epileptic seizures. However, continuous or frequent stimulation in this region may cause various neurological symptoms including speech impairment, sensory impairment, involuntary motion, memory loss and depression. Also the foci can originate in a number of areas of the brain, not just the neocortex, including the cerebral cortex, primary motor cortex, and premotor cortex hippocampus, to name a few. Thus, stimulating only the neocortex may not be effective.
While several approaches have been employed for the localization of epileptic foci using electroencephalogram measurements (EEG), these have largely relied on surface electrodes, which also have drawbacks. These include very weak signals when epileptogenic foci are located in deep brain tissue, when there are two or more foci which can cancel each other out (due to the dipole nature of the signal) or closed field foci (due to the foci being located in a sheet of non-parallel tissue. Other drawbacks with surface electrodes include the tendency of the various tissue layers which overly the foci (e.g., the meninges, bone, skin, etc.) to spread out the signal over a larger layer of the scalp making localization difficult and the fact that dipoles generated by the foci can be oriented parallel or obliquely to the electrodes causing phase reversal and false localization of the signal. Many of these same issues, including difficulties in localization and phase reversal, can also occur for an implanted electrode. These issues can make detection of pre-seizure events leading to a seizure even more difficult since the magnitude and duration of aberrant neural-electric activity during the pre-seizure event can be reduced compared to an actual seizure.
Accordingly, a need exists for devices and methods for detecting seizure or pre-seizure events/states so that acute treatment (e.g., drug or stimulation) can be delivered to prevent the seizure and/or minimize its effects.